Collimators are used in a wide variety of equipment in which it is desired to permit only beams of radiation emanating along a particular path to pass a selected point or plane. Collimators are frequently used in radiation imagers to ensure that only radiation beams passing along a direct path from the known radiation source strike the detector, thereby minimizing detection of beams of scattered or secondary radiation. Collimator design affects the field-of-view, spatial resolution, and sensitivity of the imaging system.
Particularly in radiation imagers used for medical diagnostic analyses or for non-destructive evaluation procedures, it is important that only radiation emanating from a known source and passing along a direct path from that source through the subject of examination be detected and processed by the imaging equipment. If the detector is struck by undesired radiation, i.e., radiation passing along non-direct paths to the detector, such as rays that have been scattered or generated in secondary reactions in the object under examination, performance of the imaging system is degraded. Performance is degraded by lessened spatial resolution and lessened energy resolution, both of which result from noise in the signal processing circuits generated by the detection of the scattered or secondary radiation rays.
Collimators are positioned to substantially absorb the undesired radiation before it reaches the detector. The collimator comprises a relatively high atomic number material placed so that undesired radiation strikes the body of the collimator and is absorbed before being able to strike the detector. In a typical detector system, the collimator includes barriers extending outwardly from the detector surface in the direction of the radiation source so as to form channels through which the radiation must pass in order to strike the detector surface.
Some radiation imaging systems, such as computerized tomography (CT) systems used in medical diagnostic work, use a point (i.e. a relatively small, such as 1 mm in diameter or smaller) source of x-ray radiation to expose the subject under examination. The radiation passes through the subject and strikes a radiation detector positioned on the side of the subject opposite the radiation source. In a CT system the radiation detector typically comprises a number of one-dimensional arrays of detector elements. Each array is disposed on a flat panel or module, and the flat panels are typically arranged end to end along a curved surface to form a radiation detector arm. The distance to a given position, typically the center of the panel, on any one of the separate panels is the same, i.e., each panel is at substantially the same radius from the radiation source. On any given panel there is a difference from one end of the panel to the other end in the angle of incidence of the radiation beams arriving from the point source. In any system using a "point source" of radiation and panels or modules of detector elements, some the radiation beams that are desired to be detected, i.e., those traveling directly from the radiation source to the detector surface, strike the detector surface at some angle offset from vertical.
For example, in a common medical CT device, the detector arm is made up of a number of panels or modules, each of which has dimensions of about 32 mm by 16 mm, positioned along a curved surface having a radius of about 1 meter from the radiation point source. Each panel has about 16 separate detector elements about 32 mm long by 1 mm wide arranged in a one-dimensional array, with collimator plates situated between adjoining elements and extending outwardly from the panel to a height above the surface of the panel of about 8 mm. As the conventional CT device uses only a one-dimensional array (i.e., the detector elements are aligned along only one axis), the collimator plates need only be placed along one axis, lengthwise between each adjoining detector element. Even in an arrangement with a panel of sixteen 1 mm-wide detector elements adjoining one another (making the panel about 16 mm across), if the collimator plates extend perpendicularly to the detector surface there can be significant "shadowing" of the detector element by the collimator plates towards the ends of the panel. This shadowing results from some of the beams of incident radiation arriving along a path such that they strike the collimator before reaching the detector surface. Even in small arrays as mentioned above (i.e. detector panels about 32 mm long), when the source is about 1 meter from the panel and the panel is positioned with respect to the point source so that a ray from the source strikes the middle of the panel at right angles, over 7.5% of the area at the end detector elements is shadowed by collimator plates that extend 8 mm vertically from the detector surface. Even shadowing of this extent can cause significant degradation in imager performance as it results in nonuniformity in the x-ray intensity and spectral distribution across the detector module. In a one-dimensional array, the collimator plates can be adjusted to be slightly offset from vertical to compensate for this variance in the angle of incidence of radiation from the point source.
Advanced CT technology requires use of two-dimensional arrays, i.e., arrays of detector elements on each panel that are typically arranged in rows and columns. In such an array, a collimator must separate each detector element along both axes of the array. The radiation rays from the point source to each detector on the array have different orientations, varying both in magnitude of the angle and direction of offset from the center of the array. Setting up collimator plates along two axes between each of the detector elements in two dimensional arrays would be extremely time consuming and difficult. Additionally, arrays larger than the one-dimensional array discussed above may be advantageously used in imaging applications. As the length of any one panel supporting detector elements increases, the problem of the collimator structure shadowing large areas of the detector surface becomes more important.
Accordingly, one object of the present invention is to provide a highly focused collimator for use in imagers having point radiation sources and an efficient method to readily fabricate such a collimator.
Another object is to provide a readily-fabricated collimator for use with two-dimensional detector arrays used in conjunction with a point radiation source.